A tire-pressure monitoring system (TPMS) is a system for monitoring the air pressure of tires of a vehicle. When the TPMS detects that one of the tires of the vehicle is improperly inflated, an indicator light on the instrument panel illuminates to warn a driver about the tire. TPMS may be indirect or direct.
Indirect TPMS monitors information available outside of the tire to indirectly determine the air pressure. Some indirect TPMS relies on individual rotational speeds of the tires. If one tire is rotating faster than the other tires, then the tire has a smaller diameter and thus is likely underinflated.
Direct TPMS uses pressure sensors mounted either inside or on an outer surface of each tire. Pressure sensors mounted inside the tires communicate using wireless short-range signals.
Neither indirect nor direct TPMS monitors tire wear, and indirect TPMS may not detect if all tires are underinflated rather than only one tire. Direct and indirect TPMS require additional sensors for the vehicle. Determining tire wear by mileage count is often imprecise because not all miles wear down tires equally. Visual inspection of tires can be unreliable and relies on humans to perform the inspection.